A flash memory device is a type of electrically erasable programmable read-only memory (EEPROM) and is used for non-volatile storage of data. Flash memory is being increasingly used to store execution codes and data in portable electronic products, such as computer systems.
A typical flash memory comprises a memory array having rows and columns of memory cells. Each of the memory cells is fabricated as a field-effect transistor having a control gate and a floating gate. The floating gate is capable of holding a charge and is separated by a thin oxide layer from source and drain regions contained in a substrate. Each of the memory cells can be electrically programmed (charged) by injecting electrons from the drain region through the oxide layer onto the floating gate. The charge can be removed from the floating gate by tunneling the electrons to the source through the oxide layer during an erase operation. Thus, the data in a memory cell is determined by the presence or absence of a charge on the floating gate.
Typically, the control gate of each memory cell of a row of the array is connected to a line (called a word-line) having a common voltage (word-line voltage), and the drain region of each memory cell of a column of the array is connected to a line (called a bit-line) having a common voltage (bit-line voltage). Flash memories currently have a typical operating voltage (Vcc) in the range of about 1.5 to 5 volts. A high voltage (or programming voltage), however, is usually required for programming and erase operations in a flash memory. This high voltage is often 8 volts or higher. During a programming operation, electrons may be injected onto the floating gate by applying the high voltage to the control gate, e.g., via the word-line, and about one-half of the high voltage to the drain region, e.g., via the bit-line, while the source region is grounded. Other mechanisms for adding or removing charge from the floating gate are also known.
In many applications, programming a flash memory cell includes pumping up the voltage at the control gate (word-line voltage) to a to a program-verify or read voltage, e.g., at or slightly above a threshold voltage (Vt) of an erased cell, using a charge pump. When the word-line voltage reaches the program-verify voltage, a program-verify is performed to determine if the memory cell is programmed. This usually involves sensing a current of the cell. For example, if the cell current is above a reference value, indicating that the Vt of the cell is below that of a programmed cell, the cell is not programmed. If the cell current is below the reference value, indicating that the Vt of the cell is above that of an erased cell, the cell is programmed.
If the cell is not programmed, the algorithm enters a program mode, and the charge pump pumps the word-line voltage from the program-verify voltage to the programming voltage, while another charge pump pumps the voltage at the drain region (bit-line voltage) from about zero to about half the word-line voltage, initiating programming of the cell. However, the rise time of the word-line voltage in going from the program-verify voltage to the programming voltage is relatively long because the charge pump is typically designed to initially pump the word-line voltage to the program-verify voltage relatively slowly to avoid overshooting the program-verify voltage. This increases the programming time because the rate of programming is reduced during the rise time due to the relatively low voltages during early parts of the rise time.
In some applications, programming times can be reduced by using a high external programming voltage (Vpp), e.g., about 12 volts. For example, during manufacturing, Vpp is applied to the word-line without using a charge pump to reduce the rise time and thus the programming time. Vpp is also regulated down and applied to the bit-line for supplying the bit-line voltage. However, many flash-memory programming processes do not support an external Vpp. Moreover, using a high Vpp can sometimes cause various internal circuits of the flash memory to break down.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternatives for programming flash memory cells.